1. Field of the Invention
This invention relates to a protective layer thermal transfer sheet and a printed article having an image on which the protective layer of the sheet is transferred.
2. Description of the Related Art
Conventionally, images such as tone images and mono-tone images including characters and symbols have been formed on a base material through a thermal transfer system. With respect to the thermal transfer system, a heat-sensitive sublimation transfer system and a heat-sensitive melt transfer system have been widely used.
Of these system, in the heat-sensitive sublimation transfer system, a thermal transfer sheet, formed by placing a dye layer made by melting or dispersing a sublimable dye serving as a colorant in a binder resin on a base material, is used, and with this thermal transfer sheet being superposed on an image receiving sheet, energy in accordance with image information is applied to a heating device such as a thermal head so that the sublimable dye contained in the dye layer on the thermal transfer sheet is transferred to the image-receiving sheet; thus, an image is formed thereon.
This heat-sensitive sublimation transfer system makes it possible to control the amount of dye transfer on a dot basis in response to the quantity of energy to be applied to the thermal transfer sheet. Therefore, this system has the advantages of forming a superior tone image and of easily forming characters, symbols and the like.
By the way, various cards, such as social position certificates, driver's licenses, member evidences and ID cards, have been widely used. Various pieces of information, which clarify social positions, etc. of the owner, are recorded in these cards. In particular, with respect to ID cards and the like, the photographic image of face is important in addition to character information such as an address and a name, and the image needs to have a property (durability) of being less susceptible to change or degradation for a long time.
At present, with respect to information recording in these cards, the heat-sensitive sublimation transfer system has been adopted. In the tone image and mono-tone image formed by the heat-sensitive sublimation transfer system, however, since transferred dye is present on the surface of the transfer-receiving medium, the images are poor in resistance, such as solvent resistance and plasticizer resistance. For this reason, attempts have been made to improve the durability of images in cards that require durability, such as ID cards and the like on which face photographic images are recorded.
Japanese Patent Application Laid-Open No. 2003-080844 (patent document 1) has disclosed a protective layer thermal transfer sheet, in which a protective layer having a thermal transferring property is placed on at least one portion of one of the faces of a substrate sheet and the protective layer, which forms the outermost surface after a thermal transferring process, on the substrate sheet side contains a copolymer made from at least two or more components selected from methyl methacrylate, methacrylic amide and methacrylic acid.
The transfer sheet, disclosed in patent document 1, is used for transferring the protective layer onto a transfer-receiving medium (image-receiving sheet) with an image formed thereon so as to improve the durability of the image.
By using the transfer sheet described in patent document 1, the durability of the image is improved to a practically usable level; however, there have been demands for imparting further superior durability to an image (protected image) on which the protective layer is formed. For example, in the case when the protected image is made in contact with a soft vinyl chloride film containing a plasticizer for a long period, it is desired that the image should not be transferred on the film so as to prevent the image from degradation.
When a printed article (for example, driver's license) is carried in a pocket, the article is susceptible to forces such as a twisting force and a bending force, and even in such a case, it is desirable to maintain the protective layer less susceptible to cracks and chips. When cracks, chips or the like occur in the protective layer, these cause degradation in the image quality. When printed articles are kept in a bag such as a vinyl chloride sheet bag, a plasticizer contained in the vinyl chloride sheet tends to reach an image through the cracks and chips to cause degradation in the image. Moisture, lipids, such as sweat, and the like, which enter through cracks and the like, also cause degradation in the image quality.
Some printed articles (for example, ID cards) are susceptible to physical friction on the protective layer when they are passed through detection machines and the like, and it is desirable to improve the adhesion of the protective layer to the transfer-receiving medium so that the protective layer is not separated easily even in such cases.